1. Field of the Invention
The present invention is directed to a thermal battery including a transition metal fluoride cathode, a lithium aluminum anode and a fluoride electrolyte where the lithium aluminum anode includes a predetermined amount of salt.
2. Background
Thermally activated electrochemical cells are used extensively in military applications because of their relatively long shelf life and compactness, and their ability to withstand vibration and shock. The electrolyte is solid under normal storage conditions and does not conduct electricity. When the battery and/or electrolyte is heated by, for example, igniting a built-in pyrotechnic heat source, the electrolyte changes to the molten state and becomes conductive to ionically connect the electrodes to provide the desired electromotive force.
Thermal batteries have been used since World War II as primary reserve power sources for ordnance. They have long shelf lives, high reliability, and proven safety. With the advent of lithium/Iron disulfide chemistry in the 1970""s, improved energy and power densities were obtained. Today, lithium/Iron disulfide is still the state-of-the-art electrochemistry for thermal batteries. Nevertheless, batteries with higher energy and power densities are desired for advanced weapon systems.
In an effort to improve the energy density of a thermal battery, it is desirable to use transition metal fluoride cathodes as a replacement for the conventional iron disulfide.
LiAl/CuF2 (anode/cathode) cells have produced 227 Wh/kg. This exceeds the specific energy of state-of-the-art cells with iron disulfide by nearly 40%. The copper fluoride cells average 2.44 volts when discharged at a current density of 200 mA/cm2.
A publication entitled xe2x80x9cThermal Battery for a Space Based Interceptor Rocketxe2x80x9d authored by J. M. Embrey and M. E. Bolster is directed to work on metal fluorides in the late 1980""s. Some of these fluoride systems offer high voltage and high theoretical energy versus lithium in alkali metal fluoride salt electrolytes. Recently, screening tests were performed to determine the best metal fluoride candidates as discussed in a publication entitled xe2x80x9cFluoride Based Cathodes and Electrolytes for High Energy Thermal Batteriesxe2x80x9d. Iron(III) fluoride and silver(II) fluoride cathodes have observed open circuit potentials of 3.65 and 4.59 volts versus lithium in LiFxe2x80x94KF eutectic electrolyte. These metal fluorides have good theoretical capacity and energy. However, they are powerful oxidizing agents so that reaction with cell materials is a problem.
Copper(II) fluoride was the best performing metal fluoride of the screening tests. It has 3.05 volts open circuit and is much less reactive with the cell and battery materials. An object of the invention is to provide an improved thermal electrochemical cell having an improved operating characteristics.
The present invention is directed to a thermal electrochemical cell having a LiAl anode with a salt additive in the range of 10 wt % to 40 wt %, a transition metal fluoride cathode and an alkali metal fluoride salt mixture electrolyte. Preferably, the cathode is CuF2 and the electrolyte is selected from the group consisting of LiFxe2x80x94KF, LiFxe2x80x94KFxe2x80x94NaF (FLiNaK), LiFxe2x80x94RbF, LiFxe2x80x94KFxe2x80x94RbF, LiFxe2x80x94NaFxe2x80x94RbF, and LiFxe2x80x94KFxe2x80x94RbF.